Environmental stresses have an impact on bacteria.

E. coli O157:H7 has become a pathogen of concern because of its association with several serious outbreaks of foodborne illness, including those associated with undercooked ground beef. The organism possesses no unusual heat resistance. However, environmental stresses can increase its resistance to heat. You have to take into account the influence of environmental stresses when designing thermal processes that provide protection against E. coli O157:H7 or other foodborne pathogens.

The stress associated with a sublethal heat shock, which induces the rapid synthesis of heat-shock proteins, is receiving considerable attention primarily because of the importance of heat treatment to food safety. Microbial cells in which heat-shock proteins are synthesized acquire additional tolerance to a second heat challenge that would normally be lethal to them. Heat-shocked cells have to be heated twice as long as nonheat-shocked cells to achieve the same extent of lethality.

USDA-ARS scientists have found that heat shocking E. coli O157:H7 in ground beef can increase the heat resistance of the organism. They recommend establishing guidelines so that prophylactic measures can be adopted, and that environmental stresses such as heat do not make bacteria better able to survive thermal processing that would normally be considered adequate. The investigators are developing predictive models targeted at heat inactivation and cooling products at rapid rates.

The microbial safety of thermally processed foods depends on ensuring that potential foodborne pathogens likely to be present in foods are killed during heating. Thus, the heat-shock response and induced thermotolerance are significant to processors in products that are heated to lower rather than higher temperatures. Heat-shocking conditions may be encountered in minimally processed, refrigerated foods of extended durability, such as sous-vide foods. A slow heating rate, long come-up times and low heating temperatures used in making sous-vide cooked foods expose microbial cells to conditions similar to heat shock, making these cells more thermotolerant.

Thermotolerance also may be a concern in meat products kept on warming trays before final heating or reheating, or when there is an interrupted cooking cycle during processing due to equipment failure. Researchers have to take into consideration increased heat resistance caused by heat shock when designing thermal processes to ensure product safety. The heat treatment should be sufficient to inactivate the thermotolerant foodborne pathogens. The refrigeration and rapid heating of products will limit the development of thermotolerance during the pasteurization process.

Further information. Vijay Juneja, USDA-ARS Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038; phone: [phone]; fax: [phone]; email: [e-mail].